The invention relates to broadband antennas and more particularly to monopulse spiral antennas that are capable of receiving incoming signals over a bandwidth of an octave or more, operating in a plurality of different excitation modes and responding to different senses of polarization.
A primary use of such spiral antennas is for tracking, and specifically angle of arrival (AOA) measurement. For such systems, the antenna is called upon to operate in a plurality of different modes simultaneously or separately, in order to develop sum and difference patterns and/or to selectively receive simultaneously or separately, two different orthogonal senses of polarization. While the antenna configuration is specifically designed to support the different modes of excitation and to receive the different polarizations required for these special circumstances, the feed point impedance of the antenna can differ significantly between modes.
The problem of impedance matching becomes more pronounced as the number of modes and therefore the number of spiral arms is increased. It can be shown that at least three arms are required to provide a single polarization sense set of sum and difference patterns as used in AOA measurement systems. Similarly, at least five arms are required for an orthogonally polarized AOA measurement system. However, an even number of arms are more convenient for interfacing with mode forming networks. So four arm spirals are used for single polarization sense AOA systems and six or eight arm spirals for orthogonal polarization sense AOA measurement systems.
When the number of spiral arms exceeds four, the inherent characteristic impedance of the antenna becomes significantly higher (due in part to a lower interarm capacitance), than the 50 ohms commonly used as characteristic impedance feed lines and as input impedance for mode forming networks. The effect of this increase in the antenna impedance is especially significant when the antenna is operated in the sum beam mode.
While it is possible to design a separate antenna impedance matching device (transformer) to provide a fairly close match to a 50 ohm transmission line for one of its operating modes, the large shift to a different impedance in another antenna operating mode results in a significant impedance mismatch. As a compromise, the transformer may be designed to match the average impedance of the various operating modes to the 50 ohm transmission line and mode forming network impedance. In either case, an impedance transformer is required for each arm of the spiral which can have large insertion losses and can cause phase and amplitude imbalances between arms, especially at higher microwave frequencies if the antenna is designed for multioctave operation.
The difficulty of matching a monopulse spiral antenna impedance to the network mode forming impedance is unique to spiral-type, broadband antennas. Another kind of broadband antenna known as the log periodic monopole array has a configuration which enables its impedance to be adjusted by changing its height over a ground plane. However, the log periodic monopole array has other shortcomings not exhibited in the spiral antennas of the planar and shallow cone angle type, including unequal E- and H-plane patterns and phase center motion as a function of frequency. These characteristics result in uneven sensitivity to incident polarization and illumination, and contribute to defocusing losses when the antenna is used as a feed in a parabolic reflector. For these reasons, the planar and shallow cone angle spiral antennas have been preferred despite the problem in matching the antenna impedance to the mode forming network.
Prior attempts to decrease the impedance of multiarm spiral antennas in order to achieve a better match to the mode forming network have primarily involved the reshaping of the antenna arms by increasing the width of the conductors in the plane or cone of the spiral, and decreasing the edge-to-edge gap separating such conductors. The antennas are constructed with spiral arms lying in a common plane, or on the surface of a cone, and in such a configuration, the interarm capacitance can be increased (and thus the antenna impedance decreased) by enlarging the width of the conductor arms relative to the free space or dielectric gap that exists between adjacent edges of the arms. It has been found however that the impedance of the antenna cannot be sufficiently decreased before insurmountable manufacturing problems are encountered due to impractically small gaps between the conductor arms. Moreover, such small interarm spacing produces inefficient radiation of energy from the antenna.
Broad-band, monopulse spiral antennas of the type discussed above are generally disclosed in U.S. Pat. No. 3,229,293, issued to J. H. Little, et al.; and No. 3,344,425, issued to James E. Webb, administrator NASA. Orthogonally polarized, broadband monopulse antennas are generally disclosed in U.S. Pat. No. 3,681,772 issued to P. Ingerson and No. 4,243,992 issued to B. Lamberty et al.
Also pertinent to the background of the invention is U.S. Pat. No. 2,856,605, issued to E. R. Jacobsen. U.S. Pat. No. 2,856,605 discloses a spiral-type of antenna in which a pair of dipole arms are each formed in the shape of a conductive strip of increasing width as a function of length and interwound so as to create a "distributed capacitance" that is intended to achieve a "substantially mean input impedance" over the bandwidth of the antenna. The purpose and teaching of this inventor are to replace a "lumped" capacitance at the outboard turns of the spiral arms, by a distributed capacitance. The capacitance is uniformly spread over the plurality of turns of the antenna arms and the magnitude of capacitance per wavelength is constant from the center of the antenna to the outermost turns of the spiral elements. Since an N-arm spiral is capable of operating in N-1 independent modes, such a two-arm spiral is not intended for, nor capable of, functioning as a broadband, monopulse antenna system for determining direction of arrival by multimode sum and difference operation or for separately receiving different orthogonal senses of polarization. It is capable of a single mode of operation only (N=2, N-1=1 mode), that is, a sum mode, and a single sense of circular polarization only, which corresponds to the wrap direction of the elements. For reasons discussed above, a two-arm spiral antenna is impractical for direction of arrival sensing and multipolar operation.